The present invention relates to chalcogenide amorphous semiconductor materials, their application to the making of current control devices and methods of making the same. In this context, chalcogenide materials are construed as materials containing at least 2 percent of one or more of the chalcogenide elements, sulfur, selenium or tellurium.
Chalcogenide amorphous semiconductor materials have been used in recent years for the manufacture of two types of switching devices, one of which devices is sometimes referred to as a threshold switch device and the other of which is sometimes referred to as a memory switch device. Such devices are disclosed in U.S. Pat. No. 3,271,591 to S. R. Ovshinsky granted Sept. 6, 1966. When a film of such chalcogenide material is sandwiched between or extends between two suitable electrodes, the application of electrical pulses of the correct energy time profile can cause the structure to display either a high or a low resistance, with a resistance ratio at least from about 10.sup.3 to 10.sup.4. In its high resistance or relatively non-conductive state, these devices have resistivities in the range from about 10.sup.3 to 10.sup.12 ohm-centimeters, and in their low resistance or conductive states they commonly have resistivities in the range of from about 10 to 10.sup.-.sup.6 ohm-centimeters.
The threshold switch devices are driven into a low resistance or conductive state by a voltage in excess of a given threshold voltage value and remain in their conductive states until the current flow therethrough drops below a given holding current value. Examples of chalcogenide materials used in threshold switch devices include compositions of (a) 40% tellurium, 35% arsenic, 18% silicon, 6.75% germanium and 0.25% indium; and, (b) 28% tellurium, 34.5% arsenic, 15.5% germanium and 22% sulfur.
Memory switch devices are driven into a low resistance or conductive state by a voltage in excess of a given threshold voltage value and remain in their conductive states even after all sources of energy are removed therefrom, and are resettable to their relatively non-conductive state by application of a reset pulse, as explained in the aforesaid U.S. Pat. No. 3,271,591. The voltage pulse which sets a memory device-forming material is generally a pulse of milliseconds duration. A reset pulse is a very short current pulse lasting generally less than about 6 microseconds in duration. Memory switch semiconductor materials are vitreous semiconductor materials which are reversibly changed between two stable structural states generally between relatively dis-ordered or amorphous and relatively ordered crystalline states. Their compositions are at the border of the glass regions, and are generally binary compositions of tellurium and germanium with germanium comprising generally greater than 10% of the composition or compositions like this including additional elements of group V or VI of the periodic table. Examples of memory material compositions are (a) 15% (atomic) germanium, 81% tellurium, 2% antimony and 2% sulfur; and (b) 83% tellurium and 17% germanium.
The useful chalcogenide compositions used in making threshold and memory switch devices have been materials which have had p-type conductivity, as generally indicated by a positive thermopower or Seebeck coefficient measurement. (The designation of a material as an n-type conductivity material in the semiconductor art generally refers to a material which has electrons as majority current carriers rather than holes characteristic of p-type conductivity materials.) The conductivity type of chalcogenide materials used to make threshold or memory switch devices was not considered to be of any real significance, since the threshold and semiconductor switch devices referred to are not p-n junction devices like transistors where successive layers of different conductivity type are associated together to form an amplifier device or the like. However, it has been recently proposed to make two and three terminal devices from chalcogenide compositions to form rectifiers and transistors, where the particular conductivity type of the material involved can be of significance.
While it was sometimes heretofore stated that it was believed that n-type chalcogenide materials existed or could be made prior to the present invention, specific n-type chalcogenide compositions were not reported and chalcogenide materials were generally thought to be p-type materials. Also, while some experimental work has heretofore been carried out in testing various electrical characteristics of germanium-tellurium binary compositions of varying percentages of these two elements, measurements were not reported from which it could be determined whether any of these compositions were of the p or n conductivity type. However, applicants have taken measurements of such binary compositions and found that within the range of compositions of these two elements useable in making memory switch devices (namely where tellurium constituted 35-95% of the binary composition), the material is of the p-type. It was discovered, however, that this binary composition is of the n-type for tellurium percentages in the range of from near 0 to near 35%. The resistivities of these n-type germanium-tellurium compositions are of from very low to modest magnitudes, making them suitable as layers for junction devices where low resistivities are desirable. (However, all switch device-forming chalcogenide compositions will generally be in a useful low resistance state to form satisfactory two and three terminal p-n junction devices in their switched conductive state even when their unswitched high resistance state has a high resistivity.)
A useful feature of some of these new n-type amorphous chalcogenide compositions is that some of them have significantly lower resistivities at room temperature than resistivities reported before by other workers testing resistivities of amorphous germanium of the n conductivity type. Thus, it was discovered that some of these new alloys of germanium with small amounts of chalcogenide materials were fabricated having lower resistivities than germanium itself.